Pure chromium



p 6, 1.960 R. s. DEAN ETAL 2,951,794

PURE CHROMIUM Filed May 12, 1958 56m, D S. DEAN FRANK X M CANLEY n J W ATTORNEY INVENTORS v PURE CHROMIUM Reginald S. Dean, Hyattsville, and Frank X. McCawley,

Cheverly, Md., assignors to Chicago Development Corporation, Riverdale, Md., a corporation of Delaware Filed May 12, 1958, Ser. No. 734,413

1 Claim. (Cl. 20435) This invention relates to pure chromium, articles made from it and methods for the preparation of same. It has for its aim the production of chromium by electrorcfining in the form of coarse idiomorphic crystals and crystal intergrowths, and the production by electrolytic means of alloys of chromium with othermetals. It also has for its aim, the production of pure chromium and chromium alloys in the form of ductile adherent coatings on metals.

This application is a continuation-in-part of Serial No. 648,888, now Patent No. 2,863,765, filed March 27, 1957.

In the known art, chromium has been produced by a variety of methods. Chromium made, for example, by electrolysis of aqueous solution and by magnesium reduction of chromium chloride contains oxygen which must be eliminated by other procedures as stated by Sully, Chromium, p. 55, Batterworths, London, 1954. The same authority states that chromium obtained from amalgam is very finely divided.

Pure chromium has been produced by dissociation of the iodide in the form of a coating on a wire. Crystals are not idiomorphic or macroscopically distinct.

Reduction of CrCl with hydrogen produced chromium powder of only 99% purity according to Maier in US. Bureau of Mines, Bull. 436,109 (1942).

Electrolysis of fused salts has been disclosed by F. Krupp in UK. Patent 197,837 (1922). Kroll et al., US. Bureau of Mines Rep. 4752, December 1950, found that metallic chromium produced in this way contained 3.6% oxide.

In one embodiment of our present invention, we produce chromium as macroscopic idiomorphic crystals of hexagonal form containing less than .01% oxygen; a group of such crystals is shown in the single figure; the hexagonal form is clearly shown by the outline of the crystals. The atomic arrangement of these crystals is shown by X-ray spectrometry to be body centered cubic; the crystals are slowly attacked by dilute sulphuric and hydrochloric acid but are readily passivated by dilute nitric acid.

The chromium content of the crystals is more than 99.99% Cr. They are somewhat ductile at room temperature and become highly ductile at slightly elevated temperatures.

In a preferred process for the production of these chromium crystals, which we will describe in detail in examples, we pass a direct current from a comminuted crude chromium anode to an iron cathode in an electrolyte of molten sodium chloride ha ing dissolved therein chromium chlorides and metallic sodium. The first step of such a process is the precipitation of small chromium crystals in a salt layer at the surface of the cathode. These crystals consolidate into a layer of consolidated chromium forming a ductile, highly protective plate on the metal cathode. The large idiomorphic crystals, and crystal intergrowths of our invention are attached to the cathode by the salt layer containing dispersed fine chromium crystals.

The production of this plate is one of the objects of 2,951,794 Patented Sept. 6, 19 60 our invention, and the plated objects are one of the articles of our invention.

The plate consists of micro-crystals and is highly deformable and ductile. This layer of consolidated metal has an electrical resistance of 3.6 microhm cm. It is highly protective to steel in salt water.

The chromium crystals of our invention can also be used for the production of alloys by are melting of consumable electrodes containing crystals of chromium of our invention and other high purity metals such as crystal intergrowths of pure titanium and zirconium.

The current density range on both anode and cathode for the production of chromium crystals is unusually great. The cathode current density on the original cathode may be 10-500 amperes/sq. ft. and on the anode, from .2 l-50 amperes/sq. ft.

The method of our invention is applicable to the production of chromium alloys of iron, nickel and cobalt by the simple expedient of adding chlorides of these metals to the electrolyte to maintain the necessary concentration of these metals throughout the electrolysis. This. concentration is a function of the cathode current density. In the case of iron, which is less noble than nickel or' cobalt, the iron content of the bath can be maintained by an auxiliary anode of iron with a higher current density; than that on the chromium anode.

Having now described ourv invention in its general terms, we will illustrate it by examples.

Example I In this example, we take low carbon ferrochrome: analyzing:

Percent;

Chromium 71.4- Iron 28.6 Silicon .27 Carbon .031 Insol. 1.23

We comminute this material to pass an 8 mesh screen,

and place it in a foraminous nickel basket concentrically disposed around a steel rod in an electrolytic cell provided with an argon atmosphere and having an electrolyte of molten NaCl in which there is dissolved 5% Cr as chromium chloride, average valence 2.05 and .1% metallic sodium.

We prepare this electrolyte by melting a mixture of CrCl and NaCl placing it in the cell described and passing a direct current from chromium in the anode basket to the steel cell wall at 5 amperes until 1.2 faradays of current have. been passed for each 52 grams of chromium present. We then place ferrochrome in the basket.

The approximate surface of the ferrochrome in the anode basket is sq. ft. and the immersed surface of the initial cathode rod is .5 sq. ft.

We pass a direct current of 100 amperes for 3 hours at 800 C.

The cathode rod with adhering material is then removed from the bath and cooled, without access to air, 290 grams of macroscopic chromium crystals having a hexagonal outer form are removed from the cathode and. washed with 1% HCl to remove salt. After dryingthese crystals had a hardness of 126 D.P.H. and were: cold ductile.

They had less than .0l% oxygen and no other detectable impurities. The composition of the bath was not; changed by the passage of current.

The cathode rod from which the crystals were removed was coated with a layer of salt 10 mils thick containing dispersed fine chromium crystals. This layer Was: scrubbed ofi and the rod was found to be plated with. a layer 3 mils thick of ductile chromium in a continuous: non-porous plate protective to steel in salt water.

Example II In this example, we proceed as in Example I, except as follows, the electrolyte is 65% SrCl 35% NaCl in which is dissolved 8.9% Cr as chloride having an average valence of 2.16 and .6% dissolved alkalinous metal; the temperature was 600 C.; the cathode is an aluminum rod.

The results after 3 hours passage of current are identical with thoseof. Example I including the formation on the aluminum rod of a non-porous ductile plate.

Example III In this example, We take crude chromium in pulverulent form and mix it with CrCl in proportions to form Cr and add this to NaCl in such proportion that the total Cr content is 10% by weight. We heat this mixture to 800 C. and place it in a cell as described in Example I With a comminuted chromium anode and pass a current of 5 amperes for 3 hours. The resulting bath analyzes 9.8 total soluble chromium; average valence determined by ferric sulphate solution 2.08; sodium by hydrogen evolution in acidified ferric sulphate solution 0.5%.

.We then place a molybdenum cathode in the cell having an immersed surface of 2 square feet. The chromium in the anode basket has a surface of 1000 square feet. We pass a current of 500 amperes for 3 hours at 800 C.

The molybdenum cathode is then removed from the bath and cooled in argon. 1410 grams of macroscopic chromium crystals are removed from the cathode.

The crude chromium in the anode analyzed 1.6% oxygen, 3% iron, balance substantially chromium. The crystals obtained from the cathode after washing analyzed less than .01% O and less than .0l% Fe with no determinable amount of other impurities.

Example IV This example is carried out exactly as Example I, except that high carbon ferrochrome analyzing 58% Cr, 7% carbon, 1% silicon, balance substantially iron. The result is the same as in Example I.

4 Example V In the example, we proceed as in Example I, except that an auxiliary anode of iron is placed between the nickel basket and the cathode. This anode has a surface of .1 sq. ft. The cathode product is uniform and analyzes 35% iron. The crystals are highly magnetic.

Example Vl We proceed as in Example I, except that during electrolysis, we add gradually 103 grams of cobalt as anhydrous and cobaltous chloride. adherent to the cathode analyse 30% cobalt, balance chromium. The alloy crystals are cold ductile and can 7 be melted to an alloy having high strength at elevated temperature.

What is claimed is:

The method of producing a non-porous chromium plate on a. steel cathode electrochemically protective to said steel which consists in passing a direct current from a comminuted crude chromium anode to said steel cathode in an electrolytic cell having an electrolyte consisting of at least one alkalinous chloride in which is dissolved from 1l0% chromium as chloride having an average valence of 2.052.2 and about 0.1% alkaiinous metal as determined by hydrogen evolution in acidified ferric chloride solution, at a temperature of about 800 C. and a current density on the original cathode surface of 10-500 amperes/sq. ft. and on the anode from .01- amperes per square foot until a plate on the cathode of at least one mil thickness is formed, then continuing the electrolysis to produce a layer of salt containing fine chromium crystals up to 10 mils thick surmounts the plate, then cooling the cathode and adherent deposit in an inert atmosphere and removing the layer of salt and fine crystals by scrubbing with water.

References Cited in the file of this patent UNITED STATES PATENTS 1,821,176 Driggs Sept. 1, 1931 2,752,303 Cooper June 26, 1956 2,834,727 Gullett May 13, 1958 2,863,765 Dean et al Dec. 9, 1958 2,874,454 Gullett Feb. 24, 1959 The resulting crystals 

